The present invention relates to minimizing the buildup of slag in high-temperature areas of a furnace system. This is accomplished by adding to the ash-laden flue gas which produces the slag buildup certain substances which reduce the rate of accumulation by altering the chemical and physical characteristics of the ash under prevailing temperatures. The physical properties of the solid ash which does accumulate are such that the deposit is readily removed from the furnace surfaces rather than producing large accretions of buildup on those surfaces.
Boiler slagging occurs when hot, molten or semi-molten ash particles strike and adhere to furnace surfaces in the radiant section of the boiler. Fouling occurs in the lower temperature convective sections of the boiler when volatile constituents in the ash such as the alkali oxides condense and collect further ash which then sinters into a hard mass. It is to slagging, and more particularly to the minimization thereof, that the present invention is directed.
Slagging occurs in all furnace systems. Some slap buildup on the inner walls of the furnace is often desirable in order to provide thermal insulation and thus minimize heat loss through the furnace walls, but excessive slag buildup tends to clog the furnace and/or produce excessive temperatures within the furnace.
While slagging problems may occur in connection with all types of fuel, as a practical matter, the problems are particularly acute when coal is the fuel in question, and it is to the control of coal slagging problems that the present invention is directed. The use of coal as a source of energy, particularly for the generation of electric power, can be expected to increase in the future, for economic reasons (the increasing cost of fuel oil) as well as for long term energy policy reasons (making the nation less dependent on imported sources of energy). Coal is not a uniform product--coal supplies from different natural sources have different compositions, different combustion characteristics, and different propensities toward producing slag, and in particular have different ash contents and different ash compositions. Ash content and the composition of the ash greatly affect the tendency to slag production.
The ash produced during combustion is classified according to the way in which it is removed from the boiler. Fly ash is that portion which is entrained in the flue gas and subsequently removed by air pollution control devices such as electrostatic precipitators. Bottom ash, as its name implies, remains in the boiler and is removed through the bottom of the firebox. Boilers are designed to remove this ash as either a molten, flowing slag or a dry, friable ash. Those designed for molten slag removal are known as wet bottom boilers while those designed for solid ash removal are known as dry bottom boilers. It is of critical importance to efficient boiler operation that the ash be in the physical state for which the unit was designed. U.S. Pat. No. 4,057,398, for example, deals primarily with the problems associated with high melting ash in wet bottom boilers. The instant invention is directed towards substantially the opposite problem: to wit, the slagging tendencies of low melting ash in dry bottom boilers. Hence the solution of the slagging problems presented here involves a different approach from that taught in the aforementioned U.S. Pat. No. 4,057,398.
Many coal consumers are being forced to switch from their normal supplies to alternate sources simply because of the increase in demand for coal. In addition, increasingly stringent environmental regulations regarding sulfur dioxide emissions have caused many coal users to switch from their normal high-sulfur coal to low-sulfur supplies. In many cases these alternate coal supplies are completely different from the design coal with regard to ash fusion temperature, ash composition, etc. Substitution of coal with ash characteristics significantly different from those for which a boiler was designed can give rise to problems such as a slagging.
Boiler design is an important factor in determining whether or not deposits will form when a particular fuel is burned. Many factors are considered in designing boilers capable of handling the ash characteristic of a particular coal. The objective of such design considerations is to optimize the combustion process and reduce deposits to a minimum, thus maximizing the efficiency of extraction of energy from the fuel. Maximum heat extraction requires careful control of the relative quantities of heat absorbed through the waterwalls, superheat tubes, reheat tubes, etc. The control of deposits in these areas is, therefore, critical. The correct thermal balance in a deposit-free boiler is achieved by controlling factors such as furnace volume and the relative surface areas of the superheat and reheat sections. Effective heat utilization must be attained without promoting deposit formation, and the final design for a boiler represents the product of effective heat utilization and minimization of deposits.
Slag is a deposit build up by impaction of molten or semimolten ash on a surface where it can solidify. Particles of ash are normally molten when they exit the flame zone of a boiler. If the melting point of the ash or the rate of solidification is too low, the particles will not have sufficient time to solidify before impinging on a boiler surface. When this occurs, the molten or plastic ash adheres to and solidifies on the surface giving rise to a slag deposit. In designing a boiler to burn a particular coal, the characteristics of the ash can be accommodated by adjusting the size of the furnace and thermal profile such that fly ash can solidify before contacting a surface. For any other coal the presence or absence of slag will depend upon the dynamics of the boiler which determine whether the ash is solid or molten by the time it reaches a surface.
Slagging has a major effect on boiler operation. As the plastic ash particles impact on upper furnace walls and superheat tubes, significant accumulations of hard slag can result. Accumulations result in partial blockage of the gas flow, necessitating reductions in boiler load. In some cases, slag may build up to the extent that damage to lower waterwall tubes can result from dislodging heavy accumulations. Another effect of slagging in the boiler is insulation of the waterwall tubes in the radiant section which leads to thermal imbalance within the boiler. As the waterwalls become insulated by slag accumulation, heat transfer efficiency decreases, and temperatures in the superheat section become excessively high. The energy waste and monetary penalty resulting from these conditions are obvious.
Certain mechanical modifications such as increased sootblowing can result in reduction of deposits, but seldom result in complete alleviation of the problem. The most universal method of correcting slagging conditions is simply to reduce load. At reduced load, temperatures throughout the boiler are lower, molten ash solidifies faster, and slagging conditions are minimized. Boiler derations of 10% of total generating capacity due to slagging are not uncommon, and in some cases deration of 25% or more is required to avoid boiler shutdown. While reduction in load is an effective solution to the slagging problem, it is economically undesirable for the obvious reason that equipment is not being used to maximum capacity. Power purchased to meet requirements is more expensive than generated power.
In order to achieve maximum boiler efficiency without damage to steam tubes, superheat and reheat steam temperatures are generally maintained at about 1000.degree. F. When a coating of slag insulates the waterwalls, heat transfer in the furnace area is reduced, which results in higher flue gas temperatures in the superheat/reheat section of the boiler. The correspondingly high superheat and reheat steam temperatures are reduced to the optimum value of about 1000.degree. F. through the use of attemperating sprays. Attemperating spray is water or low-temperature steam which is introduced directly into the superheat or reheat tubes and produces a cooling effect on outlet steam.
In a typical slagging boiler little or no attemperating spray is required when the unit starts up, since waterwall tubes are clean and heat absorption in the furnace area is efficient. As waterwalls begin to slag, excessively high steam temperatures in the superheat and/or reheat tubes necessitate the use of attemperating spray. As slagging of the waterwalls continues to increase, attemperating sprays automatically increase. Finally, when the maximum sprays are reached and steam temperatures are still too high, thermal balance in the boiler can only be restored by reducing load and shedding slag. Slag separates from the tube surfaces upon reducing load and cooling the boiler because of the difference in contraction rates of metal and slag.
The length of a cycle from start-up with clean waterwalls until deration becomes necessary to shed slag varies with the severity of the problem, but can be as short as several hours. It is usually difficult, if not impossible, to observe the buildup of waterwall slag while the boiler is in operation. Even where observation is possible, this technique does not provide a quantitative measure of slag buildup. Since attemperating spray usage is proportional to the degree of waterwall slagging, however, it serves as a useful measure of the severity of the slagging condition. Attemperating spray usage is normally recorded continuously and serves as a quantitative record of the development of a slagging condition from start-up to final deration.
One commonly used method of assessing slagging potential of coal ash is the ASTM fusion test. In this test small cones of ash particles are heated and the temperatures of various degrees of deformation noted. However, recent investigations brought the realization that the observable softening temperature of the ash was in many instances not the controlling characteristic thereof insofar as slag production is concerned. The ash particles, it was discovered, after being made molten, would upon recooling remain molten to temperatures well below their normal melting temperature, and they would then strike the boiler surfaces while still molten and harden into amorphous glassy solids which would be very tough and removal-resistant. The fact that they supercooled before solidifying meant that deposits would remain molten within the furnace for longer periods of time, thus permitting additional particles to be caught by the molten deposit mass, thereby increasing the speed and amount of buildup, while the dense state of the solidified mass made it exceedingly difficult to remove.
U.S. Pat. No. 4,372,227, entitled "Method of Reducing High Temperature Slagging In Furnaces," notes that there are certain substances which, when combined with the molten ash, will have the characteristic of minimizing the super-cooling tendency of the untreated particles and thus accelerate the particle solidification into a crystalline material rather than an amorphous, glassy mass, and theorizes that those substances served as nucleating agents during the cooling of the ash particle, causing that particle to solidify at a temperature quite close to its normal melting temperature and fostering its solidification into a crystalline form. More specifically, the aforementioned U.S. Pat. No. 4,372,227 teaches that the addition of particles of alumina, silicon carbide or aluminum nitride are extremely effective in that regard.
The object of the present invention is to provide a method of ameliorating high-temperature slagging conditions which result from impaction on the boiler interior surfaces of molten or semi-molten ash particles in flue gas resulting from the combustion of the fuel.
Another object is to provide such a method which is economical because it utilizes a synergistic mixture as the conditioner added to the flue gas.
A further object is to provide such a method which utilizes as the conditioner compounds which would not appear useful for that purpose according to ash fusibility tests.